As a backlight light source of a liquid crystal display device, a cold cathode discharge tube is generally used, which is lit and driven by an inverter transformer that generates high voltage. An inverter transformer is, for example, structured with a primary winding and a secondary winding formed at a winding portion of a bobbin, as well as with winding ends connected to a plurality of terminals disposed in a protruding manner at a terminal-fixing portion, and equipped with a core so as to form a closed magnetic circuit. Since a relatively large current flows through the primary winding, an electric wire with a wire diameter of equal to or more than 0.1 mm (referred to as a thick wire) made by stranding a plurality of polyurethane-covered wires is used. On the other hand, although a high voltage is generated in the secondary winding the amount of current that flows through the secondary winding is small, so a single polyurethane-covered wire (referred to as a thin wire) with a wire diameter of equal to or less than 0.1 mm (e.g. the wire diameter of a copper wire is about 0.03 to 0.04 mm) is used. Each winding end made from the above wire materials are bound to the terminals and soldered. Although soldering is performed by dipping (immersing) into molten solder, in general, the dipping temperature and dipping duration differ for a thick wire and a thin wire described above, and therefore dipping is performed under their respective optimal conditions.
Problems rarely occur with the thick stranded wires, but with the thin single wires, in their connections to the terminals, there are two major problems that are likely to cause failures. One of them is that when bending deformation occurs due to an external force applied to a tip portion of the terminal, deformation also occurs at a binding portion of the wire material at the foot so that the wire material locally extends, and thus leads to a risk of a broken wire. The other problem is that, since the wire material is thin, a break is apt to occur from being affected by heat stress during dip soldering.
A method for solving the problem by improving the shape of the terminal itself is proposed (refer to Patent document 1) to solve the former problem regarding a wire break associated with an external force applied to the tip portion of the terminal. This is a technique for preventing a break in the wire material by forming a constriction or making the diameter small at the foot of the terminal at a position ahead of the binding portion of the winding end (on the side opposite the bobbin), so that when an external force is applied to the proximity of the terminal tip portion, bending occurs at the constriction or the small diameter portion and deformation does not occur at the portion of the terminal for binding the winding end. However, such a terminal shape will be a factor that leads to a reduction in strength and an increase in cost will occur since processing is required for the terminal.
Regarding the latter problem on a wire break caused by heat effects during dip soldering, a method for solving the problem by devising the structure of a terminal-fixing portion is proposed (refer to Patent document 2). Such a structure has ribs formed on both sides of an upper terminal-fixing portion, a foot portion of a draw-out groove for drawing out a winding end positioned substantially in line with a top surface of the rib, the winding end that is to be drawn out to the outside from the draw-out groove is drawn out from the foot portion of the draw-out groove, further hung on the top surface of the rib, and then bound to the terminal. In this way, an attempt is made to prevent a wire break caused by the winding end sinking into the mold melted by heat effects during dip soldering.    Patent document 1: Japanese Patent Application Laid-open No. 11-176658    Patent document 2: Japanese Patent Application Laid-open No. 2001-345222